cfq-iosched: fix RCU problem in cfq_cic_lookup()

cfq_cic_lookup() needs to properly protect ioc->ioc_data before
dereferencing it and also exclude updaters of ioc->ioc_data as well.

Also add a number of comments documenting why the existing RCU usage
is OK.

Thanks a lot to "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> for
review and comments!

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 4df3f05..d01b411 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1142,6 +1142,9 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
        kmem_cache_free(cfq_pool, cfqq);
 }
 
+/*
+ * Must always be called with the rcu_read_lock() held
+ */
 static void
 __call_for_each_cic(struct io_context *ioc,
                    void (*func)(struct io_context *, struct cfq_io_context *))
@@ -1197,6 +1200,11 @@ static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic)
        cfq_cic_free(cic);
 }
 
+/*
+ * Must be called with rcu_read_lock() held or preemption otherwise disabled.
+ * Only two callers of this - ->dtor() which is called with the rcu_read_lock(),
+ * and ->trim() which is called with the task lock held
+ */
 static void cfq_free_io_context(struct io_context *ioc)
 {
        /*
@@ -1502,20 +1510,24 @@ static struct cfq_io_context *
 cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
 {
        struct cfq_io_context *cic;
+       unsigned long flags;
        void *k;
 
        if (unlikely(!ioc))
                return NULL;
 
+       rcu_read_lock();
+
        /*
         * we maintain a last-hit cache, to avoid browsing over the tree
         */
        cic = rcu_dereference(ioc->ioc_data);
-       if (cic && cic->key == cfqd)
+       if (cic && cic->key == cfqd) {
+               rcu_read_unlock();
                return cic;
+       }
 
        do {
-               rcu_read_lock();
                cic = radix_tree_lookup(&ioc->radix_root, (unsigned long) cfqd);
                rcu_read_unlock();
                if (!cic)
@@ -1524,10 +1536,13 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
                k = cic->key;
                if (unlikely(!k)) {
                        cfq_drop_dead_cic(cfqd, ioc, cic);
+                       rcu_read_lock();
                        continue;
                }
 
+               spin_lock_irqsave(&ioc->lock, flags);
                rcu_assign_pointer(ioc->ioc_data, cic);
+               spin_unlock_irqrestore(&ioc->lock, flags);
                break;
        } while (1);
 
@@ -2134,6 +2149,10 @@ static void *cfq_init_queue(struct request_queue *q)
 
 static void cfq_slab_kill(void)
 {
+       /*
+        * Caller already ensured that pending RCU callbacks are completed,
+        * so we should have no busy allocations at this point.
+        */
        if (cfq_pool)
                kmem_cache_destroy(cfq_pool);
        if (cfq_ioc_pool)
@@ -2292,6 +2311,11 @@ static void __exit cfq_exit(void)
        ioc_gone = &all_gone;
        /* ioc_gone's update must be visible before reading ioc_count */
        smp_wmb();
+
+       /*
+        * this also protects us from entering cfq_slab_kill() with
+        * pending RCU callbacks
+        */
        if (elv_ioc_count_read(ioc_count))
                wait_for_completion(ioc_gone);
        cfq_slab_kill();